Annealing properties of copper



July 28, 1959 s. CARLEN EYTAL 2,397,107

ANNEALING PROPERTIES OF COPPER Filed Jan. 26, 1956 g/t lead Recrystallization temperature C jmwbm i United States Patent 2,897,107 ANNEALING PROPERTIES OF COPPER Siilve Carln, Vasteras, and Sven Anders Lundquist,

Skelleftehamn, Sweden, assignors to Bolidens Gruvaktiebolag, Skelleftehamn, Sweden, a joint-stock company limited of Sweden Application January 26, 1956, Serial No. 561,611

Claims priority, application Sweden August 24, 1955 3 Claims. (Cl. 14813.2)

The present invention relates to a method of improving the annealing properties of electrolytic copper, that is copper of the grade obtained by electrolytic refining processes, or copper of a corresponding purity, but which does not show satisfactory properties as to capability of being annealed.

' Products of electrolytic copper or copper of a corresponding purity, which are to be used particularly for electrical conduction purposes in the form of wire, strips, bars or the like, are substantially manufactured by cold working as to the final shaping operations, the reduction of the cross section being effected by drawing, rolling or forging. On deformation of copper at lower temperatures, such as room temperature, the physical properties of the metal are changed insofar as the tensile strength and the hardness of the metal are changed according to the increasing degree of cold working. Because of this the resistance to deformation will be greater if the reduction as to cross section is continued or if finished products are subjected to shaping operations, such as bending. Therefore it is often necessary to take care of this condition of the metal by annealing, the original tensile strength properties of the metal being re-obtained by recrystallization. Efiecting a recrystallization process in the copper metal necessitates heating of the metal to a certain temperature, such as '150-500 C., during a certain time, such as 120-10 minutes. The higher temperature, the shorter time and vice versa. Substantially the time and temperature necessary in order to effect said recrystallization will be a function of the degree of the cold working on the one hand as well as the kind and the contents of impurities dissolved in the copper co-crystals on the other hand.

Generally great amounts of impurities will result in a higher recrystallization temperature. From the technical point of view it is desirable that the annealing step be carried out at as low a temperature and during as short a time as possible. Accordingly the purity grade of the copper must be a high one and furthermore substantially constant so as not to allow great changes as to recrystallization properties of the copper metal to occur.

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, 2 the heat treatment are governed by the properties of the lacquer. i 1

For these and other reasons of a purely operative technical nature it will be necessary in many cases to discard. copper materials. which cannot be satisfactorily annealed in this way. 7

It has been found that the annealing capacity of the copper can be predetermined. This is done by determining the recrystallizationtemperature of the material under "certain, hereinafter described, standardized conditions. The method. has been worked out and described by Smart and Smith (Trans. AIME 143 (1941), page 272), and in certain details modified and standardized by the present inventors and is on the whole connected to the annealing conditions existing in practical operation. The method consists in the following steps:

(1) A material is heated to 800 C. and rolled to a cross-sectional diameter of (2) .The rolled wire thus-obtained is annealed at 700 C. .for 20 minutes. I 3) The rolled wire is rapidly cooled down and dipped in a diluted aqueous solution'of sulfuric acid and then rinsed'and allowed to dry.

(4) The rolled wire is cold-drawn to a cross-sectional diameter'of 2.25

(5) Pieces of said wire are heat-treated at different temperatureseg. 175, 200, 250 and 500 C. for a period of 1 hour and the tensile strength is determined. The tensile strength is also determined for pieces of said wire which have not been heat-treated.

(6) 'The relation between tensile strength and heat treatment temperature is plotted in a graph and the temperature is chosen that represents the tensile strength "corresponding to the interval occurring halfway between entirely hard and entirely soft wire. This temperature is characterized as the recrystallization temperature of the material. 3

The combination of a very large number of determinations of recrystallization temperatures as to different items and different kinds of electrolytic copper shows that the recrystallization temperature lies between a minimum of about 175 C. and a maximum of about 250 C. At the same time it has been proved by tests that such kinds of copper as have recrystallization temperatures exceeding about 200 C., and particularly 210-220 C. do not fulfill the requirements as to anncaling capacity for certain specified purposes as stated above. 7

Existing variations as to the recrystallization temperature of. the-copper depend upon variations in the degree of purity. In this case the presence of certain elements as impurities, even when present in small proportions,

Copper products manufactured by cold working are generally annealed in a furnace of special construction. This annealing step may be carried out either subsequent to the cold working step or as an intermediate annealing step, before the final dimension has been reached. In this process wire and strip materials are annealed in the form of wound-up rings. It has been found that certain maximum annealing temperatures must not be exceeded, since otherwise the recrystallization structure of the metal will be too coarse, and moreover the different windings ofthe wire and strip rings will tend to adhere to each other to a certain extent.

In particular cases the annealing is carried out in connection with the application of an insulating lacquer coating by burning. In this operation the conditions for Element: C. increase Ag 1 Ni I s r 0 As 7 V 16 Sb ,21 s- 46 Se 65 Te 55 The values areapplicable to oxygemcontaining copper:

V 3 Electrolytic copper has a high tains seldom more than 100 grams of impurities per ton, if the oxygen of cast materials of tough pitch copper is not included. The varying influence of different elements when present as impurities is attributed eg to different degrees of absorption of said elements in solidified copper crystals while forming a solid solution.

With a view to studying the influence of different impurity elements as to the recrystallization temperature of the copper the present inventors carried out numerous tests. Said tests also included tests with elements of the kind that have not been previously examined, but which elements are often present in commercial .copper, viz. bismuth and lead. It has previously been considered that these two elements are insoluble in copper co-crystals and thus should be inert with regard to the recrystallization temperature of the copper. It has been proved by degree of purity and con- We found that a hot rolling of wire bar copper to wire having a diameter of 9.6 mm. corresponds to the thermal pre-treatment obtained by annealing at 700 C. (This pre-treatment was introduced as a standard in the test treatment for estimating the recrystallization temperature and will be found in the method heretofore described with reference to the above point 2.) The influence of the lead additions necessary to provide a desired effect on the thermal pre-treatment is thus the following: for annealing temperatures of about 700-800 C. an amount of 20-100 g./t. is used and for annealing temperatures of 600 C. and below that value an amount of 10-70 g./t.

is used. For extremely high annealing temperatures, such tests that at such low amounts as 1-5 grams per ton as those exceeding 850 C., there is a risk that the effect of the lead addition will be lost and such high temperatures should thus be avoided in carrying out the process according to the present invention.

The specific effect of the lead addition for lowering the recrystallization temperature of the copper is thus higher for lower contents of sulfur in the copper and higher for'lower annealing temperatures prior to cold working. Of course, it may be considered to be an advantage, both an economic as well as a quality point The present invention is characterized in that to copper tion temperature to about 200 C. or below. The amounts of lead that are necessary for attaining this effect, are independent of the composition of the copper as far as the content of the metallic impurities aswell as the content of oxygen are concerned but are dependent on the sulfur content and moreover are dependent on the thermal pretreatment to which the copper is to be subjected prior to the cold-working step.

Thus, in order to obtain the desired effect by the addition of lead (by desired effect is meant a lowering of the recrystallization temperature of the copper from above about 210 C. to below 200 C.) further factors, such as the sulfur content of the copper as well as the thermal treatment of the copper prior to cold working, should be taken into account. The influence of these two factors on the necessary addition of lead has been estimated by tests carried out by the present inventors.

In electrolytic copper or copper of corresponding purity and grade the content of sulfur is normally below 15 g./t. and seldom exceeds 25 g./t. For sulfur contents up to 15 g., an amount of 20-40 g./ t. lead is used; for sulfur contents of 15-25 g./t., an amount of -100 g./t. lead is used.

The thermal pre-treatment of the copper prior to cold working is generally determined by the method used for production, in which method the cooling step subsequent to casting or hot rolling is of decisive importance. In most cases, at least in the production of wire products, a hot rolling step is generally carried out, the condition of the copper prior to cold working being dependent on the result of said step. For the sake of simplicity the condition of the copper obtained by such a thermal pretreatment will be defined by one statement of temperature, it being presumed, however, that identical conditions are obtained either by an annealing treatment at constant temperature or by the technical hot treatment.

i of view, to be able to lower the amount of added lead by combining the addition of lead with a further treatment of the copper material. A low content of sulfur in the copper is of importance in production since the higher content of sulfur is considered to result in deleterious effects on the quality of the copper, not the least of which is the increasing effect per se of the sulfur on the recrystallization temperature of the copper. The thermal pre-treatment of the copper prior to cold working, which is given by the temperature curve of the copper material during the rolling in the normal production of wire, may suitably be completed with a further annealing treatment at a lower temperature, such as 600 C. or below, but not below 400 C.

The results obtained which show more specifically the effect of the lead content on the recrystallization temperature as well as with respect to thesulfur contents of the copper and the thermal pre-treatment of the copper are combined in the accompanying diagram, in which the different curves correspond to sulfur contents and annealing temperatures according to the following:

Curve 1.Copper having a sulfur content of 10 g./t. and heat treated at 700 C. This curve shows a maximum effect at 30 g./t. and then a reduction of the effect with greater additions of lead.

Curve 2.-Copper having a sulfur content of 10 grams per ton and heat treated at 600 C. This curve shows a maximum influence at 40 grams lead per ton and then a constant effect'irrespective of further lead additions.

Curve 3.-Copper having a sulfur content of 20 grams per ton and heat treated at 700 C. This curve shows no influence up to 40 grams lead per ton, and then a reduction with further lead additions.

Curve 4.-Copper having a sulfur content of 20 grams per ton and heat treated at 600 C. This curve shows no influence up to 30 grams lead per ton, and then a reduction with further lead additions.

The curves reveal that according to the present invention the stated limits of lead content are only based on a maximum effect on the annealing properties to be obtained. 1

It is ordinarily considered that high lead content are detrimental to the mechanical properties of copper. This undesired result is, however, obtained only at higher proportions of lead than those involved in this invention, i.e. at amounts of more than about grams per ton, especially grams per ton.- On the other hand, electrolytic copper generally contains lead to an extent less than 10 grams per ton. Thus, by the additions proposed herein such high lead content are never obtained that would have any detrimental effect on the mechanical properties of the copper. Neither has any deterioration of the electrical conductivity of the copper been observed as a result of the practice of the present invention.

Having now particularly described and ascertained the nature of our said invention and which manner the same is to be performed, we declare that what we claim is:

1. In a method for improving the annealing properties of electrolytically pure copper which, due to its containing sulfur and metallic elements as impurities, has a recrystallization temperature of above 200 C., the improvement which comprises adding to said copper a quantity of lead in an amount sufiicient to lower the recrystallization temperature to below 200 C. and then heat treating the copper to a temperature between about 400 C. and 600 C.

2. In a method for improving the annealing properties of electrolytically pure copper which, due to its containing sulfur and metallic elements as impurities, said sulfur being in a quantity of up to 25 grams per ton of copper, has a recrystallization temperature of above 200 C. the improvement which comprises adding between about 100 grams of lead per ton of copper whereby the recrystallization temperature is lowered to below 200 C. and then heat treating the copper to a temperature between about 400 C. and 600 C.

3. In a method for improving the annealing properties of electrolytically pure copper which, due to its containing sulfur and metallic elements as impurities, said sulfur being in a quantity of up to grams per ton of copper, has a recrystallization temperature of above 200 C. the improvement which comprises adding between about 10- grams of lead per ton of copper whereby the recrystallization temperature is lowered to below 200 C. and then heat treating the copper to a temperature between about 400 C. and 600 C.

References Cited in the file of this patent UNITED STATES PATENTS Hewitt et al. Jan. 24, 1956 OTHER REFERENCES 

3. IN A METHOD FOR IMPROVING THE ANNEALING PROPERTIES OF ELECTROLYTICALLY PURE COPPER WHICH, DUE TO ITS CONTAINING SULFUR AND METALLIC ELEMENTS AS IMPURITIES, SAID SULFUR BEING IN A QUANTITY OF UP TO 25 GRAMS PER TON OF COPPER, HAS A RECRYSTALLIZATION TEMPERATURE OF ABOVE 200* C. THE IMPROVEMENT WHICH COMPRISES ADDING BETWEEN ABOUT 1040 GRAMS OF LEAD PER TON OF COPPER WHEREBY THE RECRYSTALLIZATION TEMPERATURE IS LOWERED TO BELOW 200* C. AND THEN HEAT TREATING THE COPPER TO A TEMPERATURE BETWEEN ABOUT 400* C. AND 600* C. 